1. Field of the Invention
This invention relates to a friction element for a motor car or motor cycle disc brake and more particularly to a method and apparatus for producing such a friction element.
2. Description of the Prior Art
As shown in FIGS. 1a and 1b of the accompanying drawings, a friction element for a motor cycle disc brake conventionally includes an irregular-shaped backing plate 9 which is typically formed of mild steel and which supports a single, sintered friction pad 10 typically having a cross-sectional area of the order of 1800 mm.sup.2. By way of contrast, a friction element for a tractor disc brake conventionally includes a generally circular backing plate supporting a plurality of angularly spaced friction pads which are positioned adjacent the periphery of at least one major surface of the plate and each of which typically has a cross-sectional area of only about 130 mm.sup.2. In each case, the or each friction pad is normally composed of a sintered friction mixture which may contain copper, nickel, iron or aluminium as its major ingredient together with a variety of additives to improve the properties of the pad.
At the present time, friction elements for motor cycle disc brakes are normally produced by cold-compacting the powdered friction material into a preform and then, after locating the preform in position on the backing plate, furnace sintering the preform under pressure so as to sinter the preform into the required pad and bond the pad to the backing plate. In the case of a mild steel backing plate and a copper-based friction pad, satisfactory bonding can only be achieved at the temperatures involved in the furnace sintering operation (i.e. 700.degree.-900.degree. C.) if the backing plate is precoated normally by electrolytic plating, with a layer of a bonding aid, such as nickel or copper, the coating typically being between 0.01 and 0.025 mm thick. Although the inclusion of this precoating step enables the furnace sintering process to produce satisfactory friction elements, the process suffers from a number of practical disadvantages, in particular high capital cost and space requirements. A further disadvantage is that, due to the considerable time at an elevated temperature, the backing plate becomes annealed.
In the field of tractor disc brakes, it is known not only to use furnace sintering, but also electrical resistance heating (see British Pat. No. 1,460,592), to sinter and bond preformed friction pads to a backing plate. Moreovoer, although resistance sintering of conventional copper-based friction pads to a mild steel backing plate still requires pre-plating of the backing plate with a bonding material, such as copper or nickel, the resistance heating technique has been found to produce satisfactory friction elements without the high capital cost and space disadvantages of the furnace sintering route.